It is deemed that the period of ringing caused by switching becomes longer than the pulse width of a PWM signal in a control region in which the duty ratio of the PWM signal is decreased when a brushless DC motor is controlled in a pulse width modulation (PWM) manner, and driven according to a position-sensorless method. In this case, the rotational angle of a rotor cannot be accurately detected, which is a problem.
Patent Literature 1 discloses technology (referred to as “balanced-drive switching”) by which switching devices positioned on upper and lower arms of energizing phases in an inverter circuit are exclusively ON-OFF controlled so as to suppress a change in neutral point potential and to reduce common mode noise. According to this technology, when switching is not performed, the terminal voltage of a winding contains only an induced voltage. Accordingly, the rotational angle of the rotor can be stably detected even in the control region in which the duty ratio of the PWM signal is decreased.
In contrast, when the balanced-drive switching is adopted, a ripple current is increased in an operating region in which a motor rotates at a high speed. An increase in the capacity of a smoothing electrolytic capacitor is required to suppress the occurrence of the ripple current, and thus a circuit size is increased, which is a problem.